1. Field of the Invention
The present invention relates to a variation detection device and a method of detecting variation. More specifically, the present invention relates to a variation detection device and a variation detecting method for detecting variation of an output signal from a status detecting apparatus such as a pressure sensor and the like.
2. Description of the Related Art
In recent years, regulations with regard to safety have been intensified one after the other in Japan and United States. According to the TREAD Act (Transportation Recall Enhancement, Accountability and Document Act) which takes effect in North America, a new car that will be sold on 2006 or later is obligated to carry a vehicle tire pressure monitoring system. Therefore, it is currently considered to provide sensors in tires in order to measure tire pressure and temperature. More specifically, sensor units are mounted on valve sections of respective tires such that all four wheels can be monitored individually. Such a system has the advantages that the monitoring in high precision can be attained and the tire pressure can be monitored even during parking and stopping.
When measuring the tire pressure (air pressure of a tire of a vehicle), a sensor unit which is mounted on a valve section of the tire is typically used. Generally, a tire temperature (temperature of a tire of a vehicle) is different between a case when the vehicle is running and a case when the vehicle is stopping. The temperature of the tire of a running vehicle becomes higher than the temperature at the time of stopping due to friction between the tire and a road surface. When the tire temperature increases, temperature of the sensor unit which is mounted on the valve section of the tire also increases.
With regard to a pressure sensor for measuring the tire pressure, an output of the pressure sensor may possibly change depending on the temperature, even if the tire pressure applied to the pressure sensor is constant. Therefore, in order to accurately measure a variation of the tire pressure in a certain interval from a first time to a second time, the tire temperature (namely, the temperature of the pressure sensor) is required to be constant during the interval or to be the same at least between the first time and the second time. However, the temperature is hardly the same between the first time and the second time, when the variation of the tire pressure is measured under natural circumstances. For this reason, an error caused by the temperature difference has been conventionally corrected through a calculation (refer to National publication of the translated version of PCT application JP-2003-511287, for example).
FIG. 1 is a graph showing a temperature characteristic of the pressure sensor. The graph in FIG. 1 represents change in sensor output as a function of the temperature under a condition that the pressure is constant. For example, the constant pressure applied to the pressure sensor is 450 kPa in FIG. 1. As shown in FIG. 1, the output of the pressure sensor changes in response to the change in the temperature. More specifically, the sensor output decreases as the temperature increases. When the correction for the sensor output is carried out through a calculation, it may be required to define a function beforehand which represents a relationship between the sensor output and the temperature shown in FIG. 1. However, the function changes when the applied pressure is changed. Thus, it is difficult to prepare such functions associated with all the pressures in advance. Hence, the collinear approximation is usually applied when the correction is carried out. However, the collinear approximation causes an error to occur in the correction.
When the temperature of the circumstance under which the tire is used changes largely, the air pressure inside the tire is also changed largely. Fro example, when the tire temperature is increased, the air inside the tire expands and thereby the air pressure inside the tire is also increased. It is also required to recognize precursor of tire blowout by detecting a slight change in the tire pressure. Therefore, in order to calculate the variation of the tire pressure accurately in a tire pressure detecting system, it is necessary to consider the fact that the tire pressure is changed in accordance with the temperature variation.
It is required to use a pressure sensor of high precision in order to reduce the error of the sensor output caused by the temperature variation. However, such a high-precision pressure sensor is expensive. As mentioned above, a new car that will be sold on 2006 or later is obligated to carry a vehicle tire pressure monitoring system. When the TPMS employs an expensive circuit, the cost is increased correspondingly. Also, even in the case of the pressure sensor of high precision, the correction is not possible when the tire pressure is changed in accordance with the temperature variation.
A technique is desired which can accurately measure the variation of the tire pressure in a certain interval independent of the tire temperature at the time of the measurement. It is desired to obtain an accurate output without increasing cost.